Display devices for use in TVs, cellphones, and various other devices and optical elements such as a camera lens usually adopt antireflection techniques in order to reduce surface reflection and increase the quantity of light transmitted through them. The reason is that when light is transmitted through the interface between multiple media with mutually different refractive indices (e.g., when light is incident on the interface between the air and glass), the quantity of transmitted light decreases due to Fresnel reflection and other factors, thus deteriorating the visibility.
According to one of those antireflection techniques that has attracted a lot of attention these days, a very small uneven pattern in which the pitch of its recessed and raised portions is adjusted to be equal to or smaller than the wavelength of visible light (with λ=380 nm to 780 nm) is formed over the surface of the substrate (see Patent Documents 1 to 4). Those raised portions which form part of the uneven pattern that produces the antireflection function have a two-dimensional size of 10 nm to less than 500 nm.
This method uses the principle of a so-called “moth-eye structure”. The refractive index with respect to light that has been incident on the substrate is continuously changed in the depth direction of the recessed and raised portions from the refractive index of a medium on which the light has been incident to the refractive index of the substrate, thereby reducing reflection significantly in the wavelength range in which reflection needs to be reduced.
The moth-eye structure is advantageous because the structure not only performs the antireflection function with small incident angle dependence over a wide wavelength range but also is applicable to a lot of materials and contributes to forming an uneven pattern on the substrate directly. That is why by adopting the moth-eye structure, a high-performance antireflection film can be provided at a lower cost.
The moth-eye structure may be formed using an anodized porous alumina layer which is obtained by anodizing aluminum (see Patent Documents Nos. 2 to 4, for example).
Hereinafter, the anodized porous alumina layer which is obtained by anodizing aluminum will be described briefly. A method of forming a porous structure by anodization has attracted a lot of attention in the related art as a simple method for making nanometer-scale circular cylindrical nanopores (very small recessed portions) which are arranged regularly. If a base is immersed in an acidic or alkaline electrolytic solution of sulfuric acid, oxalic acid, phosphoric acid, or any other appropriate acid, and if a voltage is applied thereto using the base as an anode, oxidation and dissolution will advance concurrently on the surface of the base. As a result, an oxide film with a huge number of nanopores can be formed over the surface of the base. These circular cylindrical nanopores are arranged perpendicularly to the oxide film and exhibit self-organized regularity under a certain condition (including voltage, electrolyte type, and temperature). Thus, this anodized porous alumina layer is expected to be applied to a wide variety of functional materials.
In a porous alumina layer which has been formed under a particular condition, generally regular hexagonal cells have a closest packed two-dimensional arrangement when viewed along a normal to the film surface. At the center of each of those cells, there are nanopores which are arranged periodically. The cells are formed as a result of local dissolution and growth of a coating. The dissolution and growth of the coating advance concurrently at the bottom of the nanopores (that is a so-called “barrier layer”). It is known that the interval between adjacent nanopores (i.e., the distance between their centers) is approximately twice the thickness of the barrier layer and is approximately proportional to the voltage that is applied during the anodization. It is also known that the diameter of the nanopores depends on the type, concentration, temperature, and other parameters of the electrolytic solution but is usually about one-third of the size of the cell (i.e., the length of the longest diagonal of the cell when viewed along a normal to the film surface). Such nanopores of the porous alumina may form an arrangement with high regularity (or periodicity) under a particular condition, or an arrangement with a somewhat decreased degree of regularity depending on the condition, or an irregular (non-periodic) arrangement.
Patent Document 2 discloses a method of forming an antireflection film (or antireflection surface) using a stamper, of which the surface has an anodized porous alumina film.
Meanwhile, Patent Document 3 discloses a technique for forming tapered recesses with continuously changing pore diameters by performing anodization of aluminum and a pore diameter increasing process a number of times.
The applicant of the present application discloses, in Patent Document 4, a technique for forming an antireflection film using an alumina layer in which very small recessed portions have stepped side surfaces.
As described in Patent Documents 1, 2, and 4, by providing an uneven structure (macro structure) which is larger in size than a moth-eye structure (micro structure) in addition to the moth-eye structure, the antireflection film (antireflection surface) can have an antiglare function. The raised portions that form part of the uneven structure to realize the antiglare function have a two-dimensional size of 1 μm to less than 100 μm. The entire disclosures of Patent Documents Nos. 1, 2 and 4 cited above are hereby incorporated by reference.
If the anodized porous alumina film is used, a mold for forming a moth-eye structure on the surface (which will be referred to herein as a “moth-eye mold”) can be made easily. In particular, as disclosed in Patent Documents 2 and 4, if the surface of the anodized oxide film of aluminum is used as a mold as it is, the manufacturing cost can be cut down significantly, which is beneficial. The structure of the surface of a moth-eye mold which can form a moth-eye structure will be referred to herein as an “inverted moth-eye structure”.
A known method of forming an antireflection film using a moth-eye mold uses a photocurable resin. First of all, a photocurable resin is applied over a substrate. Then, an uneven surface of a moth-eye mold which has undergone a mold release process is pressed against the photocurable resin in a vacuum, thereby filling the uneven structure on the surface of the moth-eye mold with the photocurable resin. Then, the photocurable resin in the uneven structure is irradiated with an ultraviolet ray so that the photocurable resin is cured. Thereafter, the moth-eye mold is removed from the substrate, thereby forming a cured layer of the photocurable resin, to which the uneven structure of the moth-eye mold has been transferred, over the surface of the substrate. Such a method of forming an antireflection film using a photocurable resin is disclosed in Patent Document 4, for example.
As an exemplary mold release process to be carried out on a mold with a porous alumina layer which is used to make an antireflection film, Patent Document No. 5 teaches performing the mold release process by adding a fluorine-based mold releasing agent by spray coating method.
Meanwhile, Patent Document No. 6 says that in making a lens by cast molding process, if a mold release agent diluted with a solvent is applied only once and then only the solvent is applied in order to coat the mold with the mold releasing agent uniformly, the resultant layer of the mold releasing agent can have a uniform thickness.